Effect of solution composition on the recrystallisation of 1

kaolinite to feldspathoids in hyperalkaline conditions: 2

Limitations of pertechnetate incorporation by ion 3

competition effects. 4

5

Janice Littlewood1

, Samuel Shaw2

, Pieter Bots2

, Caroline L. Peacock1

, Divyesh Trivedi3

and Ian T. 6

Burke1*

7

8

1

Earth Surface Science Institute, School of Earth and Environment, University of Leeds, Leeds, LS2 9

9JT, UK. 10

11

2

School of Earth, Atmospheric and Environmental Sciences, University of Manchester, Manchester, 12

M13 9PL. 13

14

3

National Nuclear Laboratories, Risley, Warrington, Cheshire, WA3 6AS, UK. 15

16

Corresponding Author’s email: i.t.burke@leeds.ac.uk 17

18

Abstract 19

The incorporation of pertechnetate (TcO4-) into feldspathoids produced by alkaline 20

alteration of aluminosilicate clays may offer a potential treatment route for 99Tc 21

containing groundwater and liquors. Kaolinite was aged in NaOH to determine the 22

effect of base concentration, temperature, and solution composition on mineral 23

transformation and pertechnetate uptake. In all reactions, increased temperature 24

and NaOH concentration increased the rate of kaolinite transformation to 25

feldspathoid phases. In reactions containing only NaOH, sodalite was the dominant 26

alteration product however, small amounts (6-15%) of cancrinite also formed. In 27

experiments containing NaOH/Cl and NaOH/NO3 mixtures, sodalite and nitrate 28

cancrinite were crystallised (at 70 oC), with no reaction intermediates. The addition 29

of SO42- crystallised sulfatic sodalite at 40 & 50 oC, but at higher temperatures (60 30

and 70 oC) sulfatic sodalite transforms to vishnevite (sulfatic cancrinite). In 31

experiments where a pertechnetate tracer was added (at ~1.5 μmols L-1), only 3-5 % 32

of the 99Tc was incorporated to the feldspathoid phases. This suggests that the 33

larger pertechnetate anion was unable to compete as favourably for the internal 34

vacancies with the smaller OH-, NO3-, SO4

2- or Cl- anions in solution, making this 35

method likely to be unsuitable for groundwater treatment. 36

37

Introduction 38

There is a global legacy of contaminated land around nuclear facilities, specifically 39

arising from leaking storage ponds and containers (Mon et al., 2005, Perdrial et al., 40

2011, Wang and Um, 2012). At the Sellafield nuclear facility, UK, approximately 20 41

million m3 of soil may be contaminated, and 90Sr, 137Cs and 99Tc have been identified 42

as important contaminants (Hunter, 2004). In oxic environments, technetium is 43

highly mobile in the form of its pertechnetate anion, TcO4- (Burke et al., 2005), and its 44

half-life of 2.1 x 105 years (Szecsody et al., 2014) means that 99Tc is particularly 45

problematic. Hence there is a need for low cost, preferably non-invasive, techniques 46

to be developed that might increase the pace of restoration at nuclear sites. 47

Kaolinite (Al2Si2O5(OH)4, (Grim, 1968) is commonly found in sediments and soils 48

around nuclear sites (Huertas et al., 1999), and has a simple 1:1 sheet silicate 49

structure (Bauer et al., 1998). Under alkaline conditions, kaolinite dissolves, 50

releasing Al3+ and Si4+ into solution, leading to the formation of secondary 51

feldspathoid and zeolite phases (Mashal et al., 2004, Qafoku et al., 2003, Zhao et 52

al., 2004, Wallace et al., 2013). The exact phase precipitated depends on the 53

solution composition, base concentration, Si:Al ratio and temperature (Deng et al., 54

2006b). Important features of the internal structure of these neo-formed minerals, 55

such as cancrinite ([(Ca,Na)6(CO3)1-1.7][Na2(H20)2][Si6Al6O24]) (Bonaccorsi, 2005), 56

sodalite ((Na8Cl2)[Si6Al6O24]) (Bonaccorsi, 2005) and vishnevite 57

([(Na,Ca)6-xKx(SO4)][Na2(H2O)2][Si6Al6O24]) (Bonaccorsi, 2005), are channels and 58

cages, which are known to selectively incorporate guest anions/cations (Deng et al., 59

2006b, Mon et al., 2005). Alkaline altered sediments are known to incorporate 90Sr 60

and 137Cs (Choi et al., 2005, Choi et al., 2006, Chorover et al., 2008, Deng et al., 61

2006a, Mon et al., 2005, Wallace et al., 2013), however, there is no information 62

available regarding the successful incorporation of 99Tc, in the form of pertechnetate 63

(TcO4-), into alkaline alteration products. Perrhenate (ReO4

-) has been incorporated 64

into sodalite (Dickson et al., 2014, Mattigod et al., 2006) and the incorporation of this 65

negatively charged, tetrahedral anion may support the hypothesis that TcO4- could 66

be encapsulated into similar phases. This work investigates alkaline alteration of 67

kaolinite, particularly the effect of base concentration, temperature, and solution 68

composition during aging, with a view to encapsulating 99Tc, in the form of 69

pertechnetate (TcO4-), into a range of neoformed phases. Thus, potentially providing 70

a novel remediation treatment for land contaminated with 99Tc. 71

72

Material and Methods 73

Materials: 74

Two natural kaolinite samples were used as supplied; kaolinite from Fluka Chemicals 75

(Switzerland) and sample K-Ga 1b from the Clay Mineral Society (Chantilly, USA). 76

Ammonium pertechnetate was obtained from LEA-CERCA (France). All other 77

reagents (sodium hydroxide, chloride, nitrate and sulfate) were obtained from VWR 78

international (USA). 79

80

Methods: 81

In all instances, aerobic batch experiments were carried out in sealed 50 mL 82

polypropylene Oakridge tubes. Initial experiments suspended 0.4 g of dry kaolinite 83

(Fluka) in 20 mL of NaOH solution (0.05 M, 0.5 M, 5 M), at room temperature (RT) or 84

70 oC in a temperature controlled oven (5 M NaOH only). The same solid-to-solution 85

ratio was used in all subsequent experiments. Samples were shaken on a daily 86

basis. At intervals of 1, 7, 14 and 35 days (RT), and 7 and 10 days (70 oC), tubes 87

were removed from the oven. After cooling to RT, solid phases were extracted by 88

centrifugation, at 6000 g for 10 minutes. The solid phase was washed four times 89

with deionised water and dried, at room temperature, in a desiccator containing 90

Carbosorb. 91

92

Effect of solution composition and temperature: 93

Temperature effects were determined when 0.4g dry kaolinite (Fluka) was aged in 94

20mL 5 M NaOH, for 10 days at 40 oC, 50 oC, and 60 oC. In order to investigate the 95

effect of changing the anion composition on the alteration products formed, three 96

additional basic (5 M NaOH) solutions were used that contained either 1 M NaCl, 0.5 97

M NaNO3 or 4 M Na2SO4. Kaolinite (Fluka) was aged in the NaOH/Cl solution at 70 98

oC for 10 days. Kaolinite (K-Ga 1b) was aged in the NaOH/NO3 solution at 40 oC, 50 99

oC, and 60 oC for 14 days and in the NaOH/Na2SO4 solution, at 40 oC, 50 oC, 60 oC 100

and 70 oC for 10 days. Additionally, 7 day time series experiments were performed 101

at 70 oC using kaolinite (K-Ga 1b) in NaOH, and kaolinite (K-Ga 1b) in the 102

NaOH/Na2SO4 solution, with daily sample-sacrifice. 103

104

Pertechnetate sorption experiments 105

Four sets of triplicate experiment were performed where ammonium pertechnetate 106

was added at tracer concentrations (1.5 μM) to each of basic solutions described 107

above (i.e NaOH, NaOH/SO42-, NaOH/NO3

- and NaOH/Cl-) prior to the addition of 108

kaolinite (K-Ga 1b) and incubated at 70 oC for up to 72 days. At regular intervals, 109

aqueous samples were separated by centrifugation and Tc activity determined by 110

liquid scintillation counting on a Packard TriCarb 2100TR. 111

112

Sample characterisation: 113

The starting material and alkaline alteration end products were analysed by a 114

number of complimentary techniques. Powder X-ray diffraction (XRD) using a 115

Bruker D8 diffractometer. Samples (50-100 mg) were mounted on silicon slides and 116

scanned between 2o and 70o 2θ. Rietveld refinement was carried out using TOPAS 117

4.2 software (Bruker) providing quantitative analysis of the crystalline phases. (For 118

the results presented in this study, the errors were -/+10 % at the 50 % level (i.e. 119

between 40-60 % present) and -/+1 % close to the limit of detection (e.g. at the 3 % 120

level there was between 2-4 % present). N2–specific surface area was measured 121

using the BET method, with a Micrometrics Gemini V Surface Area Analyser, with 122

samples degassed for a minimum of 19 hours, at RT, with nitrogen gas prior to 123

analysis. Electron micrographs were produced using scanning electron microscopy 124

(SEM) coupled with energy-dispersive X-ray spectroscopy (EDS) using a FEI 125

QUANTA 650 FEG environmental SEM. Samples were mounted on a carbon pad 126

and coated in platinum prior to analysis. 127

128

Results 129

Preliminary investigation of alkaline alteration of kaolinite in NaOH solutions. 130

There was no evidence of new phases in the XRD patterns (not shown) where 131

kaolinite was aged in 0.05 M and 0.5 M NaOH at room temperature for 35 days. A 132

partial phase transformation to sodalite (data not shown), occurred when kaolinite 133

was aged in 5 M NaOH, however, kaolinite peaks were still dominant at day 35. 134

XRD patterns (data not shown) for the aging of kaolinite in 5 M NaOH, at 70 oC, 135

indicated that full transformation to sodalite had occurred after 10 days. Based on 136

these results, 5 M NaOH and elevated temperatures were used in subsequent 137

experiments. 138

139

Effect of time and temperature on the alkaline alteration of kaolinite in 5 M NaOH. 140

Kaolinite was aged over 7 days in 5 M NaOH at 70 oC. XRD patterns (figure 1A) 141

show that the transformation from kaolinite to sodalite started after 1 day, evidenced 142

by the sharp decrease in the main kaolinite (k) peak intensities and the formation of 143

peaks from the neo-formed sodalite (s) and minor amount of cancrinite (c). Rietveld 144

analysis of the day 1 and 2 samples indicated the presence of 8 and 15 wt% of 145

cancrinite (figure 2A), respectively. By day 3, sodalite was the dominant phase, with 146

only minor kaolinite peaks visible. There was no evidence of cancrinite after day 2. 147

Throughout the next 4 days, the kaolinite peaks further reduced as the sodalite 148

peaks increased, until only minor amounts of kaolinite were visible in the XRD 149

patterns by day 7. Rietveld analysis (figure 2A) of day 7 samples quantified 81 wt% 150

sodalite and 19 wt% kaolinite. BET (figure 4) analysis indicates an initial increase, 151

from 12 to 16 m2/g, in the surface area at day 1, followed by a consistent decrease in 152

surface area up to day 6 (7-10 m2/g). 153

Rietveld analysis of XRD data from 10 days experiments at lower temperatures 154

(figure 6C) showed 80 wt% sodalite and 20 wt% kaolinite at 50 oC, and 24 wt% 155

sodalite, 70 wt% kaolinite and 6 wt% cancrinite at 40 oC. This indicates that the rate 156

of clay transformation increased with temperature. 157

158

Effect of time and temperature on the alkaline alteration of kaolinite in 5 M NaOH + 4 159

M Na2SO4 160

XRD patterns for kaolinite aged over 7 days, in 5 M NaOH and 4 M Na2SO4 at 70 oC 161

are shown in Figure 1B. We note sharp decreases in the kaolinite (k) peak intensies 162

between the starting material and the 1 day aged sample. The decrease in the 163

kaolinite peaks, and the appearance of small peaks at 13.9º and 24.4º 2θ was 164

consistent with the transformation from kaolinite to sulfatic sodalite/vishnevite. 165

Rietveld analysis (figure 2B) of the day 1 samples quantified the neoformed 166

feldspathoids as 2 wt% vishnevite and 29 wt% sodalite. After two days aging, the 167

XRD patterns (figure 1B) showed further reductions in the kaolinite peak intensities 168

(k) together with increasing sulfatic sodalite peak intensities (s). Rietveld analysis 169

(figure 2B) for the day 2 samples showed that sulfatic sodalite had increased to 62 170

wt%, vishnevite had increased to 2.5 wt% and kaolinite had decreased to 35.5 wt%. 171

By day 3, peaks for vishnevite (v) increased in intensity (figure 1B,), and only minor 172

kaolinite peaks were visible. Throughout the next four days, the kaolinite peaks 173

further reduced and the sodalite/vishnevite (s/v) peaks became more prominent until 174

only minor amounts of kaolinite were visible in the XRD patterns by day 7. After 7 175

days, Rietveld analysis (figure 2B) showed that sodalite had decreased to 24 wt%, 176

vishnevite had increased to 53 wt% and kaolinite had decreased to 23 wt%. This 177

suggested that the kaolinite transformed to vishnevite, via a sulfatic sodalite 178

intermediate phase. No broad humps were observed in the XRD patterns at any 179

time during the reaction indicating that there were no amorphous phases detected at 180

any time during the transformations. 181

SEM images (figure 3) of the solid phases collected during the transformation show 182

the kaolinite starting material and the sulfatic sodalite/vishnevite alteration product at 183

day 2 and 7. The kaolinite starting material (figure 3A) comprised multiple layers of 184

stacked hexagonal plates approximately 20-30 µm in size. The EDS spectra of 185

kaolinite showed the presence of Al, Si and O, with the Al and Si peaks present at 186

equal intensities. After 1 day of aging, the overall crystal morphology of the kaolinite 187

had evolved (data not shown). An interlocking desert rose morphology, 188

characteristic of cancrinite (Deng et al., 2006), was observed after 2 days (figure 3B). 189

These crystallites were approximately 2.5 µm in diameter and contained O, Al, Si, 190

Na, and S by EDS analysis, consistent with formation of sulfatic sodalite or 191

vishnevite. By day 7, the sample crystal morphology comprised spheres of 192

interlocking tabular crystals (figure 3C) with a diameter of ~3-5µm. BET (figure 4) 193

analysis indicates similar change to those observed in the pure NaOH system with 194

an initial increase (12 to 15 m2/g) in surface area at day 1 followed by a consistent 195

decrease in surface area up to day 6 (4-6 m2/g). 196

XRD patterns (figure 5) in respect of the aging of kaolinite in 5 M NaOH and 4 M 197

Na2SO4, at 70 oC for 10 days, showed the presence of sulfatic sodalite and 198

vishnevite. Rietveld analysis (figure 6B) of a sample aged at 60 oC for 10 days 199

quantified the phases to be 34 wt% sulfatic sodalite, 8 wt% kaolinite and 58 wt% 200

vishnevite. After 10 days aging at 50 oC the reaction products were 69.5 wt% sulfatic 201

sodalite and 30.5 wt% kaolinite. At 40 oC the reaction products were 36 wt% sulfatic 202

sodalite and 64 wt%. This suggested that at lower temperatures kaolinite transforms 203

to sulfatic sodalite, and to vishnevite at higher temperatures. 204

205

Alkaline alteration of kaolinite in 5 M NaOH + 1 M NaCl or 0.5 M NaNO3 206

The addition of 1 M NaCl to the 5 M NaOH solution led (figure 5) to the 207

transformation of kaolinite to sodalite after 10 days aging at 70 oC. In contrast XRD 208

data shows that aging kaolinite in 5 M NaOH + 0.5 M NaNO3 for 14 days at 60 oC 209

(figure 5) led to the formation of nitrate-cancrinite. Trace amounts of kaolinite (figure 210

5) were still present after 14 days. Rietveld analysis (figure 6A) of the 14 day sample 211

quantified the phases to be 98.5 wt% nitrate-cancrinite and 1.5 wt% kaolinite, which 212

suggested that the kaolinite to nitrate-cancrinite transformation was almost complete 213

after 14 days. Experiments were also carried out at lower temperatures and showed 214

an increase in the amount of kaolinite remaining after 14 days (i.e. 8 wt% at 50 oC 215

and 31 wt% at 40 oC). This indicates that the rate of transformation from kaolinite to 216

nitrate-cancrinite increased as temperature increased, and no reaction intermediates 217

were present. 218

Pertechnetate incorporation behaviour during phase transformation 219

The % 99Tc remaining in solution (figure 7) during the aging of kaolinite in either 5 M 220

NaOH, 5 M NaOH +1 M NaCl, 5 M NaOH + 0.5 M NaNO3, or 5 M NaOH + 4 M 221

Na2SO4, was very high, ~ 95-97 %, in all instances. This suggested that only a small 222

uptake to solids (~ 3-5 % 99Tc; figure 7, inset) occurred during the first 9 days of 223

aging. Although these experiments continued for 72 days, no further uptake was 224

observed. 225

226

Discussion 227

The end products which form during alkaline alteration of kaolinite vary depending on 228

base concentration, temperature and solution composition. The lack of mineral 229

transformation during the reaction of kaolinite in 0.05 M and 0.5 M NaOH, and partial 230

transformation to sodalite in 5 M NaOH at RT, implied that there was a threshold 231

concentration effect between 0.5 M and 5 M NaOH. Full transformation at higher 232

[NaOH] is anticipated as the increased rate of a reaction is related to increases in 233

base concentration. The rate of mineral transformation is also higher at higher 234

temperatures (Mashal and Cetiner, 2010). 235

Our experiments showed that in 5 M NaOH, kaolinite started to transform to sodalite 236

at 70 oC after 1 day, with only trace kaolinite remaining after 7 days. In previous 237

work on feldspathoid precipitation from both kaolinite (Rios et al., 2009, Zhao et al., 238

2004) and Si-Al solutions (Deng et al., 2006b), amorphous and zeolitic intermediates 239

were observed before sodalite and cancrinite precipitation. However, we did not 240

observe these intermediate phases in our experiments. 241

242

Effect of solution composition on alteration product formation 243

It is thought that guest anions form ion-pairs with Na+ ions in solution, inducing the 244

nucleation and growth of the neoformed feldspathoid end-products (Zhao et al., 245

2004). As we have stated, the end products vary depending on which anions are 246

present in solution (Choi et al., 2006), and incorporation of guest ions is likely to be 247

size dependent. The anion-Na+ ion pairs must be able to fit inside the cages and 248

channels of the aluminosilicate minerals which grow around them (Barrer et al., 249

1968). In this study, the addition of sodium sulfate, sodium nitrate and sodium 250

chloride produced end products of sulfatic sodalite/vishnevite ([(Na,Ca)6-251

xKx(SO4)][Na2(H2O)2][Si6Al6O24]), nitrate-cancrinite (Na6[Si6Al6O24]·2NaNO3) and 252

sodalite ((Na8Cl2)[Si6Al6O24]), respectively. The formation of nitrate cancrinite and 253

vishnevite resulted in guest anion substitution of NO3- and SO4

2- into the ideal 254

cancrinite structure. The sodalite structure does not have the wide channels found in 255

cancrinite, only cages. These cages contain a central anion, tetrahedrally bound to 256

four cations. As the ideal sodalite structure has a central Cl- anion, the formation of 257

sodalite in the presence of sodium chloride is as anticipated (Barrer et al., 1974), 258

(Zhao et al., 2004). 259

In the NaOH/NO3- system no reaction intermediates were observed for the 260

transformation of kaolinite to nitrate cancrinite (figure 5). The formation of nitrate 261

cancrinite in high NO3- experiments has been previously reported (Buhl et al., 2000, 262

Zhao et al., 2004). In our experiments, the degree of reaction for the NaOH/NO3- 263

system was more than any of the other systems tested, with 69, 92 and 98 wt% 264

transformation from kaolinite to nitrate-cancrinite at temperatures of 40, 50 & 60 oC 265

after 14 days. 266

267

Reaction pathway for the NaOH/SO42- system 268

The transformation of kaolinite aged in NaOH/SO42- has not been reported 269

previously, therefore, this reaction pathway was studied in more detail. The XRD 270

patterns (figure 1B) observed for the alteration products are similar to those for 271

sulfate-cancrinite which was synthesised from aluminosilicate solutions (Deng et al., 272

2006b). The morphological changes, observed via SEM (figure 3), show blade-like 273

spheres at day 2 which are similar to those which have been reported in the 274

literature for cancrinite (Choi et al., 2005), however that reaction was performed at a 275

lower base concentration, (0.01 M NaOH instead of 5 M) and for a longer time period 276

(190 days compared to 7 days). The appearance of peaks for Na and for S in SEM-277

EDS analysis, which were not present in the starting material, support the formation 278

of sulfatic sodalite/vishnevite. In summary, XRD, SEM, EDS and surface area 279

evidence all suggest that the partial alkaline alteration of kaolinite started after 1 day 280

of reaction. 281

Vishnevite (sulfate cancrinite) formation was observed after 3 days of reaction. 282

Rietveld analysis (figure 6B) shows a gradual increase in the % vishnevite over days 283

4 to 7, to 53 %, with the amount of sulfatic sodalite decreasing to 24 % over the 284

same period. This is consistent with the dissolution of sulfatic sodalite and the 285

crystallisation of vishnevite. SEM images of the reaction products at day 7 (figure 286

3c) showed minor changes in morphology, in that the spheres were smaller and 287

more tabulated, which is similar to other SEM images reported for cancrinite (Deng 288

et al., 2006c). The smaller size of the day 7 spheres was reflected by a slight 289

increase in the surface area indicating some dissolution of sodalite and 290

recrystallisation to vishnevite. By 7 days 78 % of the original kaolinite had reacted 291

producing vishnevite at 70 oC (via a sulfatic sodalite intermediate). The sodalite 292

intermediate is more stable at lower temperatures evidenced by the lack of 293

vishnevite at 40 and 50 oC. 294

295

Proposed incorporation of 99Tc into alkaline altered clays 296

The potential incorporation of the technetium anion, pertechnetate (TcO4-), into 297

cancrinite or sodalite has received some support from other authors (Mattigod et al., 298

2006, Dickson et al., 2014); however, the successful incorporation of technetium into 299

feldspathoids has not yet been achieved. The results (figure 7) of the low 300

concentration pertechnetate experiments showed only a very small 99Tc uptake 301

under any of the reaction conditions studied. Up to 5 % sorption of 99Tc to alkaline 302

alteration products was observed in all instances. Using sulfate to drive the reaction 303

toward sulfate cancrinite did not produce any increased uptake over other anions 304

such as chloride or nitrate (despite the analogous tetrahedral coordination of SO42- 305

and TcO4-). TcO4

- is a relatively large anion (~2.5 Å), and in cancrinite 99Tc is likely 306

to have been incorporated into a channel rather than a cage due to size constraints. 307

Sodalite does not have channels, so the incorporation of 99Tc would have been into 308

the cages. The low 99Tc uptake could suggest that the large pertechnetate anion is 309

unable to compete favourably for the internal sites with the smaller, and therefore 310

more size appropriate, OH-, Cl-, NO3-, or SO4

2- anions (~1.4, ~1.7, ~2.0 and ~2.3 Å 311

respectively), at these low concentrations. The successful incorporation of 99Tc into 312

alkaline alteration end products such as cancrinite and sodalite could provide a 313

substantial research contribution. However, very high concentrations of 314

pertechnetate (and conversely low concentrations of other anions) are likely to be 315

required to produce significant uptake of 99Tc to alkaline alteration end products. 316

317

Conclusions 318

Kaolinite undergoes alkaline alteration in the presence of 5 M NaOH (both with and 319

without Cl-) at 70 oC to sodalite, to vishnevite if sulfate is present (via sulfatic sodalite 320

at lower temperatures), and to nitrate cancrinite in the presence of nitrate. The rate 321

of reaction was faster for the OH and OH/NO3- systems relative to the OH/SO4

2- 322

system. Up to 5 % 99Tc tracer could be incorporated into these alkaline altered 323

feldspathoids which is insufficient to consider alkaline alteration of clay minerals to 324

be a remediation strategy for 99Tc. 325

326

327

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